What would happen if everyone on earth stood as close to each other as they could and jumped, everyone landing on the ground at the same instant?

—Thomas Bennett (and many others)

This is one of the most popular questions submitted to this blog. It’s
been examined before, including by a
ScienceBlogs
post and a Straight Dope
article.
They cover the kinematics pretty well. However, they don’t tell the
whole story.

Let’s take a closer look.

At the start of the scenario, the entire Earth’s population has been
magically transported together into one place.

This crowd takes up an area the size of Rhode Island. But there’s no
reason to use the vague phrase “an area the size of Rhode Island”. This
is our scenario; we can be specific. They’re actually in Rhode Island.

At the stroke of noon, everyone jumps.

As discussed elsewhere, it doesn’t really affect the planet. Earth
outweighs us by a factor of over ten trillion. On average, we humans can
vertically jump maybe half a meter on a good day. Even if the Earth were
rigid and responded instantly, it would be pushed down by less than an
atom’s width.

Next, everyone falls back to the ground.

Technically, this delivers a lot of energy into the Earth, but it’s
spread out over a large enough area that it doesn’t do much more than
leave footprints in a lot of gardens. A slight pulse of pressure spreads
through the North American continental crust and dissipates with little
effect. The sound of all those feet hitting the ground creates a loud,
drawn-out roar which lasts many seconds.

Eventually, the air grows quiet.

Seconds pass. Everyone looks around.

There are a lot of uncomfortable glances. Someone coughs.

A cell phone comes out of a pocket. Within seconds, the rest of the
world’s five billion phones follow. All of them—even those compatible
with the region’s towers—are displaying some version of “NO SIGNAL”. The
cell networks have all collapsed under the unprecedented load.

Outside Rhode Island, abandoned machinery begins grinding to a halt.

The T. F. Green airport in Warwick, Rhode Island handles a few thousand
passengers a day. Assuming they got things organized (including sending
out scouting missions to retrieve fuel), they could run at 500% capacity
for years without making a dent in the crowd.

The addition of all the nearby airports doesn’t change the equation
much. Nor does the region’s light rail system. Crowds climb on board
container ships in the deepwater port of Providence, but stocking
sufficient food and water for a long sea voyage proves a challenge.

Rhode Island’s half-million cars are commandeered. Moments later, I-95,
I-195, and I-295 become the sites of the largest traffic jam in the
history of the planet. Most of the cars are engulfed by the crowds, but
a lucky few get out and begin wandering the abandoned road network.

Some make it past New York or Boston before running out of fuel. Since
the electricity is probably not on at this point, rather than find a
working gas pump, it’s easier to just abandon the car and steal the new
one. Who can stop you? All the cops are in Rhode Island.

The edge of the crowd spreads outward into southern Massachusetts and
Connecticut. Any two people who meet are unlikely to have a language in
common, and almost nobody knows the area. The state becomes a patchwork
chaos of coalescing and collapsing social hierarchies. Violence is
common. Everybody is hungry and thirsty. Grocery stores are emptied.
Fresh water is hard to come by and there’s no efficient system for
distributing it.

Within weeks, Rhode Island is a graveyard of billions.

The survivors spread out across the face of the world and struggle to
build a new civilization atop the pristine ruins of the old. Our species
staggers on, but our population has been greatly reduced. Earth’s orbit
is completely unaffected—it spins along exactly as it did before our
species-wide jump.